how to smooth jagged edges?
This is entirely normal.
You can turn up the resolution of the curve in the Properties editor.
Explanation: In CG, all surfaces are secretly made up of flat pieces. The trick is to get enough of them that you don’t notice. Shade Smooth fakes the way light interacts with the surface into looking more smoothed, but does not alter the geometry. If you 3D printed a model in two versions, one shaded smooth and the other shaded flat, they would come out the exact same way.